The invention lies in the field of electronic circuits. The invention relates to a circuit configuration for filtering a radio-frequency signal, the circuit configuration having a controllable-frequency oscillator and a filter with an adjustable passband.
Such filter circuits are used in tuners for television sets, for example. It is a conventional practice for an antenna signal to be mixed with a local oscillator frequency and to be converted to intermediate frequency. Before mixing, filtering is necessary to filter out the useful signal band from the broadband reception signal. The characteristic of such filtering, which may be multistage, is corrected to the local oscillator signal and, hence, to the reception signal. Filters whose passband can be shifted by a control signal are known as tracking filters. However, the filter characteristics are subject to manufacture-related fluctuations. Therefore, the filter circuit is tuned during the final test. For such a purpose, correction values are ascertained from a measurement to trim the center frequency of the filters. For example, the correction values are permanently stored in a microcontroller handling system control and, after suitable digital/analog conversion, are additively superimposed on the control signal for the filter. Such circuit technology in tuners is called digital alignment.
Conventional television set tuners process tuning voltages up to a level of 35 volts. Although modern semiconductor technologies permit such dielectric strengths to be produced in principle, the demands on such semiconductor technology are considerable. The demands on the technology of the circuit are similarly complex because digital/analog converters and adders need to be configured for the full dielectric strength of up to 35 volts.
It is accordingly an object of the invention to provide a circuit configuration for filtering a radio-frequency signal that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that, given an appropriate level of quality in the filtering, requires lower complexity for circuitry and semiconductor processes.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit configuration for filtering a radio-frequency signal, including a control signal connection for feeding in a control signal, a controllable-frequency oscillator, at least one filter having an adjustable passband for a useful signal, the at least one filter having a variable-frequency element with a first connection and a second connection, the first connection being coupled to the control signal connection, and a device for providing a correction signal, the device coupled to the second connection.
A circuit configuration for filtering a radio-frequency signal according to the invention achieves these objectives by including a connection for feeding in a control signal, a controllable-frequency oscillator whose oscillating frequency can be controlled, at least one filter having an adjustable passband for a useful signal, the filter containing a variable-frequency element having a first connection and a second connection with the first connection being coupled to the connection for the control signal, and a device for providing a correction signal, the device having a connection for the correction signal, which connection is coupled to the second connection of the variable-frequency element.
In the circuit configuration according to the invention, the correction signal acts on the tuning element, which adjusts the frequency of the filter, and effects subtraction between the control signal and the correction signal. The correction signal is supplied to the other pole of the filter""s tuning element, as compared with the control signal, in antiphase. Given the same dynamic range for the correction signal, the absolute range for the correction signal is much lower. The correction signals to be processed are restricted to the value range required only for correction. As a result, the affected circuit parts, such as digital/analog converters, need be constructed only for a dielectric strength in the value range of the correction signal, instead of for a value range for the entire tuning voltage, as was done previously. The effect achieved is, advantageously, that the circuit can be produced with a low dielectric strength using modern semiconductor technologies. Such configuration results in a considerable cost benefit with respect to circuit manufacture and, linked to this, for the entire filter circuit.
The variable-frequency element of the filter circuit is expediently a capacitance diode. The voltage present across the capacitance diode varies the variable capacitance of the capacitance diode. The control signal, which is also supplied to the variable-frequency oscillator, acts on the cathode connection. The correction signal is applied to the anode connection of the capacitance diode. In terms of the voltage drop across the capacitance diode, the control signal and the correction signal are superimposed as a difference. As a result, the absolute value of the correction signal can remain just at the value range intended for correction.
In accordance with another feature of the invention, the device for providing a correction signal is a digital/analog converter.
The correction signal, like the control signal, is provided as an analog signal. The correction signal is ascertained by measurement at the end of manufacture of the filter circuit. Expediently, the correction signal is permanently stored digitally in a nonvolatile memory of a microcontroller. The digital/analog converter converts the digital correction signal into the analog correction signal during operation of the filter circuit. Such microcontrollers and suitable digital/analog converters are sufficiently well known.
In accordance with a further feature of the invention, the at least one filter is at least three filters, at least two of the three filters has an adjustable passband, the at least three filters have useful signal paths connected in series, a first of the at least three filters and a second of the at least three filters connected downstream of the first of the at least three filters are bandpass filters having different passbands, and a third filter of the at least three filters is a coupled bandpass filter with a block for an image frequency of the useful signal.
In accordance with an added feature of the invention, at least two filters have an adjustable passband, the at least two filters have a signal path for the useful signal, the signal path has at least one capacitive element and an inductive element, and the second connection of the variable-frequency element is coupled to the signal path.
In accordance with an additional feature of the invention, the device has a correction signal connection, and the control signal connection and the correction signal connection are decoupled from the variable-frequency element by respective resistors.
In accordance with yet another feature of the invention, the at least one filter has a useful signal input coupled to an antenna connection at which a radio-frequency signal to be filtered can be received.
In accordance with a concomitant feature of the invention, the at least one filter has a useful signal output, the controllable-frequency oscillator has an oscillator output, and a mixer is included coupled to the oscillator output and to the useful signal output.
Altogether, filtering is performed by a plurality of adjustable filters connected in succession in terms of their useful signal, otherwise referred to as tracking filters. The input signal, which is provided by an antenna after suitable matching, is supplied to a first bandpass filter, whose center frequency is controllable. Connected downstream of the output is a second bandpass filter. The second bandpass filter, in turn, is followed by a coupled third bandpass filter, which also suppresses the image frequencies. The passband or stop band of the filters can be shifted. Such shifting is effected by a control signal. The three filters are expediently driven by the same control signal. The control signal is likewise used to adjust the frequency of the local oscillator for downward mixing to intermediate frequency. Each of the filters is also supplied with an individual correction signal ascertained based on a tuning measurement.
The filters themselves each include a signal path having inductors and capacitors connected in series. The anode connection of the variable-frequency element is coupled to the signal path. The control signal and the correction signals are applied to cathode and anode connections, respectively, of the capacitance diodes through respective resistors for the purpose of DC decoupling.
The filter circuit is used to isolate the useful signal from the reception signal at the antenna, so that the local oscillator signal corresponding to the adjusted reception channel is then used to perform the downward mixing to intermediate frequency. The fundamental area of application for the circuit is in the area of tuners for television sets. Because the market segment is extremely cost sensitive, the inexpensive tuner circuits that are made possible by the invention achieve a particular economic benefit.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for filtering a radio-frequency signal, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.